The Development of MS2A Reduced Ketone Resin in Painting
Conservation

Ketone Resins based on condensation reactions involving
methylcyclohexanone and/or cyclohexanone were first patented in the
1920s2 and were produced commercially
as AW2 (BASF, Germany) and MS2 (Howards, UK) from the 1930s. By the
late 1930s several hundred tons per year of these resins were sold
into the textiles and coatings industries. They were used
particularly in a range of specialty paints and varnishes as
light-fast additives which improved gloss and hardness.

The resins have a high solubility in low polarity solvents
coupled with a high refractive index. This enables the formulation
of high-solids, low viscosity varnishes which are fairly easy to
apply by brush or spray and provide attractive optical properties.
During the 1950s ketone resins were evaluated as a more stable
alternative to dammar in conservation varnishes for works of art.
Whilst fast drying rate was an advantage in many cases (though not
all) the varnish films were found to break down even in museum
conditions. A major problem associated with these resins is the
tendency of the ketone groups and double bonds to degrade in light,
resulting in yellowing and a loss of solubility over time.

In 1959 Howards, with the support of Garry Thompson, Scientific
Advisor at the London National Gallery, developed a reduced form of
MS2 in which the double bonds and ketone groups were chemically
removed by reduction with borohydride. The reduced product,
designated MS2A, was evaluated extensively as a conservation varnish
and was enthusiastically accepted. Handling characteristics and
stability were superior to MS2 and to the natural resin varnishes,
and the optical appearance of the applied varnish was described a
being beautifully soft and silky. From 1962 MS2A was produced by
Howards specifically for the conservation of works of art.

As general demand for the unmodified ketone MS2 increased, the
plant process was changed to increase production. The new MS2
product was not suitable for reduction to MS2A. In 1963 the BASF
AW2 product was examined as an alternative source of ketone resin
for reduction, and was found to be entirely acceptable. Because of
its different chemical nature (being produced from methyl
cyclohexanone admixed with cyclohexanone rather from methyl
cyclohexanone as in the case of MS2) the reduced product was subtly
different to MS2A in terms of its ease of application and
appearance, and was designated MS2B. MS2B was produced commercially
for paintings conservators until 1967. In 1967 the BASF AW2 ketone
resin was replaced by a resin derived wholly from cyclohexanone, and
known a Ketone N. The reduction product of Ketone N was useless as
a conservation varnish. (The batch process for Ketone N was
succeeded by a continuous process to produce Larapol K80.)

In view of the continuing demand from painting conservators for
MS2A, Howards agreed in 1967 to install dedicated small scale plant
equipment which utilized the traditional process and materials to
provide small amounts of MS2A exclusively for art conservation.
MS2A production could not be justified commercially. Despite
attempts by Howards and their successors Laporte to phase it out in
subsequent years, persuasion from a number of important sources
ensured its continuance. When Howards' Ilford site was closed down
in 1973, stockpiled MS2A was not promoted but was used to satisfy
ongoing demand. This continued to 1983 when MS2A stocks were
exhausted, and required Laporte to resume production from dedicated
equipment at their Widnes pilot plant facility. Again, MS2A was
not promoted, but was produced intermittently as required and sold
at the cost price of production. This amounted to £22/Kg in 1984,
rising to £375/Kg by 1992. In 1993 the Widnes pilot plant facility
was closed down, and production equipment and know-how transferred
to Linden Chemicals Ltd. (now Linden Nazareth). Linden Nazareth
have promoted MS2A, and increasing demand has stabilized the price
at Laporte's 1992 levels (approx. $600/kilo with shipping and
customs as of 3/00). Work-in-progress will enable MS2B as well to be
commercially available in the near future.

MS2A Structure and Properties

MS2A is the chemical reduction product of a complex mixture of
methylolation and condensation products formed by reactions
involving methyl cyclohexanone, methanol and their derivatives,
e.g.,

"Typical" MS2A resin has a weight average molecular weight of
1800 and a number average molecular weight of 800, and will contain
around 8 ring units per molecule. Low boiling, lower molecular
weight impurities are removed during processing by steam and vacuum
distillation, whilst ketone groups and double bonds are removed by
chemical reduction. The purified product has the following
properties. MS2A is produced as yellow granules having a high
solubility (50%w/w) in mineral spirit (white spirit). The high
solids, low viscosity varnish, which appears colourless once applied
as a thin film, rapidly reaches gel (no flow) point. As a
chemically reduced ketone resin, the main colour forming bodies
have, in effect, been removed.

Accelerated aging trials have shown that MS2A can be additionally
stabilized by the use of HALS stabilizers (Tinuvin 292, Ciba
Geigy)5. Even under abnormal aging
conditions and in cases where oxidative crosslinking is favored, the
low molecular resin is unlikely to be come insoluble6. It has been reported that
unstabilized MS2A varnishes applied to paintings almost 40 years ago
have shown no signs of aging, and remain fully soluble in white
spirit. Synthetic microcrystalline wax (Cosmolloid 80H) or bleached
beeswax have been incorporated into MS2A varnishes to enable gloss
to be modified across the extremes, from total saturation to almost
unvarnished appearance. The addition of wax also provides a
plasticising effect to MS2A varnishes, which have a brittle nature
attributable to hydrogen bonding between the many hydroxyl groups
present in the product. These aspects of MS2A varnishes are
described more fully in the following section.

Preparation, Application, and Properties of MS2A varnishes

MS2A varnish recipes are commonly based on ones devised by Mr.
Herbert Lank, reported in published articles and in personal
communications 7,8. A comprehensive review of MS2A
varnish formulations has been provided by Ms. Sarah Fisher in the
Paintings Conservation Catalog9. A
frequently used formulation is as follows10:

(A) Basic varnish for brushing or spraying: 50g MS2A
resin broken down to small granules and suspended in a muslin bag
(if desired) is dissolved in 110 mls Stoddards Solvent (white
spirits to BS 245 having less than 20% aromatics content). The
resin dissolves overnight, or more quickly with frequent stirring
and a little heating. (The final conc. is 31% weight/final
volume; 50g of MS2A plus 110 ml Stoddard equals a total volume of
160 ml. The Editor.)

This varnish may be used from a spray gun as prepared or diluted
with white spirit or odourless kerosene (4 parts to 1 part diluent)
to provide a more slowly drying varnish solution suitable as a
brushed isolating layer. (25%)

Diluted matt varnish as finishing layer: 3 parts basic
soln. (A) is mixed with 1 part basic matt varnish (B).
(24% MS2A and 1% wax) This formulation diminishes the
brittleness of the applied varnish without affecting the gloss. By
increasing the proportion of (B) to (A) the matt
effect is introduced and heightened. This is further controlled by
adjusting the spray aperature and distance from the painting, and
can be modified again by gently polishing the dried varnish with a
soft cloth.

Open/working time. MS2A may be easily worked with a brush
following application. Dried varnish may be reworked with
additional varnish application by brush to build in uneven areas.
The silky soft optical appearance is optimal in the thinnest
coats.

Cleaning. Dust which may accumulate as surface film,
often over a number of years and particularly on paintings
frequently transported for exhibitions, is easily removed by gently
washing with distilled water, followed by gentle polishing with a
soft cloth. Varnishes most affected by dust accumulation have been
reported to be those containing higher levels of wax.

Ageing. Accelerating ageing trials have shown that
oxidation/ crosslinking can occur to induce slight yellowing and
reduce solubility in white spirit11.
Such effects should occur much more slowly than in the case of
natural or ketone resins, and should be futher reduced by the
addition of 2% Tinuvin 292 (commonly used, particularly by
conservators in the USA).

In practice, MS2A varnishes examined to date, almost 40 years
after first application, and in the absence of Tinuvin 292, have
remained unchanged to the naked eye in terms of gloss and clarity,
and have remained fully soluble in white spirit with no evidence of
discoloration in removed varnish.

References

1. Information taken from the archives of Howards
of Ilford and Laporte Industries Ltd.